Semiconductor lasers, such as edge emitting lasers and vertical cavity surface emitting lasers (VCSEL) are well known, and are formed in a wide variety of configurations. Edge emitting lasers typically consist of a flat junction formed between two pieces of semiconductor material each having been treated with a different type of impurity. When a large electrical current is passed through such a device, laser light emerges from the plane of the junction region. VCSELs, on the other hand, typically include an active area disposed or sandwiched between two mirror stacks on a semiconductor substrate. The laser is activated by driving an electrical current through the two mirror stacks and the active area. This is generally accomplished by placing a first electrode across the mirror stack at one end of the laser and a second electrode across the other mirror stack at the other end of the laser. One of the electrodes generally defines a central opening therethrough for the emission of light.
In operation, a threshold level of current must be forced through the active region of the semiconductor laser for lasing to occur. The threshold level is reached when the stimulated emissions exceeds the internal losses. Upon reaching threshold, the light output rises rapidly with the current, with most of the current resulting in laser emissions.
In applications such as bar code readers, digital versatile discs (DVD), and compact discs (CD), it is necessary or highly desirable to maintain a fixed output power. In a CD, changes in light intensity of the received signal correspond to the data bits being read from the CD. Thus, it is important that the power of the source signal be maintained at a fixed or constant value so that the power of the received data bits corresponds to the actual data rather than the drift of the source signal due to environmental changes.
The power of the source signal may be maintained at a fixed or constant value through Automatic Power Control (APC). APC of semiconductor lasers allows for a constant and consistent output from the lasers. Generally, APC of edge emitting laser devices is easily achieved because edge emitting devices emit light from two ends. APC of edge emitting lasers may be achieved by using one of the light emitting ends to measure the optical power output, which is subsequently used to adjust the electrical power input to the edge emitting device and, thereby, adjusting the optical power output.
APC of VCSEL devices, however, is a much more difficult task because the VCSEL generally emits light from only a single surface, such as the bottom or the top, thus making measurement of the output and subsequent adjustment thereof a difficult task. Conventionally, the task of APC of VCSEL devices has been accomplished by positioning an optical device or devices, such as such as photodiodes, mirrors, beam splitters, or the like, in the optical path of the emission from the VCSEL device. Manually positioning the optical devices presents several problems and disadvantages such as, high manufacturing cost, lack of repeatability, and poor quality control, thus prohibiting high volume manufacturing.
It can be readily seen that conventional APC of VCSEL devices has several disadvantages and problems, thus not easily enabling their manufacture in volume applications. Accordingly, it would be advantageous to have a method and apparatus for controlling the power of semiconductor lasers, particularly VCSEL devices, that are reliable, cost efficient, and compatible with standard semiconductor processes.